[PROJ] [EXTERNAL] Re: Proj 6 API questions

[Kristian Evers]

> On 26 Mar 2019, at 06:43, Nyall Dawson <nyall.dawson at gmail.com> wrote:
> 
> On Tue, 26 Mar 2019 at 15:25, Kristian Evers <kristianevers at gmail.com <mailto:kristianevers at gmail.com>> wrote:
>> 
>> 
>> 
>>> On 26 Mar 2019, at 00:47, Even Rouault <even.rouault at spatialys.com> wrote:
>>> 
>>>> (Disclaimer: this functionality has been around much longer than I've
>>>> been involved with QGIS, so I may be missing some of the background
>>>> here)
>>>> 
>>>> QGIS exposes the choice of ellipsoid on a per-project level. This
>>>> project ellipsoid is independent of the map CRS[1][2], so a project in
>>>> WGS84 could have an ellipsoid of GRS80, and a project in Web Mercator
>>>> could use the WGS84 ellipsoid. The project ellipsoid is used ONLY when
>>>> calculating geodetic distances and areas of features. (It's also
>>>> possible to set no ellipsoid, which means distances/area calculation
>>>> is always purely Cartesian). It's also possible for users to enter a
>>>> custom ellipsoid for a project, if they have some compelling reason to
>>>> do so!
>>>> 
>>>> So we need a way to store the project ellipsoid within the project. In
>>>> current QGIS versions that's done by the ellipsoid name, from QGIS'
>>>> custom ellipsoid db. When we move to proj6 api I'm replacing this with
>>>> the unique auth:code combination.
>>>> 
>>>> Does that help clarify?
>>> 
>>> Yes, thanks. OK, I see it now in Properties/General/Measurement.
>>> Defining an ellipsoid that is not the same as the ellipsoid of the per-project
>>> CRS makes no sense to me. And I see this is what happens if opening for
>>> example a raster in UTM NAD27 in an empty project: the ellipsoid is set (or
>>> likely let to its default value of) WGS84 ! Aouch ! If it was me, I would just
>>> kill this definition of a custom ellipsoid…
>>> 
>> 
>> I absolutely agree with Even here. This behaviour is odd and not what I
>> expected as user of QGIS. Now I wonder if I have introduced errors
>> somewhere in the past because I wasn’t aware of this (probably not, I
>> mostly work with GRS80 and WGS84).
> 
> I honestly don't have any plans to change this behavior. I cannot see
> a way to remove this choice from users, as QGIS projects don't have a
> single CRS, but they DO need a single ellipsoid. The best we can do is
> make sure that a good default selection is given to users, but we
> cannot totally remove this choice.
> 

There’s only one project CRS, right? I would say that the using the ellipsoid from that is a
good default and very likely what 99% of users would want.

In case there’s a mismatch between the user-selected ellipsoid and the ellipsoid of the
map/project CRS, how do you convert between the two ellipsoids?

/Kristian

> Looking into this a bit, there is issues with the current matching of
> initial CRS to ellipsoid. I'm hopeful the move to proj 6 will fix this
> (the current code seems unable to correctly determine the ellipsoid
> for some projections).
> 
> Nyall
> 
> 
> 
> 
>> 
>> /Kristian
>> 
>> 
>>> --
>>> Spatialys - Geospatial professional services
>>> http://www.spatialys.com <http://www.spatialys.com/>
>>> _______________________________________________
>>> PROJ mailing list
>>> PROJ at lists.osgeo.org <mailto:PROJ at lists.osgeo.org>
>>> https://lists.osgeo.org/mailman/listinfo/proj <https://lists.osgeo.org/mailman/listinfo/proj>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20190326/0fecbc08/attachment-0001.html>